How can I transfer the security settings in one Group Policy Object (GPO) to another GPO?

Unfortunately, you can't export a GPO's security settings. However, you can use the Secedit command with the /export and /mergedpolicy switches to create a security template that comprises all the security policies defined on the computer. Then you can import that template into a GPO. After creating the template, maneuver in Group Policy Management Console (GPMC) to Computer Configuration, Windows Settings, and right-click Security Settings. Select Import and specify the template you just created.

This method works flawlessly for transferring settings between GPOs in the same forest but might cause a problem if your GPOs are in different forests. Security templates store SIDs for any users or groups referenced by policies in the template. A GPO that you import from another forest will recognize the well-known SIDs of built-in users and groups but not those of users or groups created by the administrator in the other forest. Any unrecognized SIDs will be ignored.

—Randy Franklin Smith